<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      * {
        border: 5px solid black;
      }
    </style>
  </head>
  <body>
    <div>div标签</div>
    <main>main标签</main>
  </body>
</html>

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